Polarization recovery system for projection displays

ABSTRACT

A waveguide polarization recovery system both polarizes the input light energy for use with an LCD imager and converts the polarity of unusable light energy to add to the illumination of the LCD imager. The compact polarization recovery waveguide system generally includes: (1) an input waveguide that provides non-polarized light energy into the system; (2) an output waveguide that receives polarized light energy from the system; (3) a polarized beam splitter that received the light energy from the input waveguide and transmits lights energy of a first polarization type and reflects light energy of a second polarization type, and (4) a wave plate that modifies the polarization of either the transmitted or reflected light energy. The polarization recovery system also generally includes one or more mirrors that are positioned as need to direct the transmitted and the reflected light energy to the output waveguide. The input and output waveguides may be shaped as needed by the projection system. For example, either one or both of the input and output waveguides may be tapered as needed to produce a desired image. In the waveguide polarization recovery system, the input and output waveguides are configured to have either an either a substantially parallel or a substantially perpendicular orientation. In another embodiment, the waveguide polarization recovery system further includes has one or more “gaps” of optically clear material positioned between the optical components to encourage the occurrence of total internal reflection that minimizes the loss of the optical energy by the system.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of application Ser. No.09/814,970, filed Mar. 23, 2001, which claims benefit of U.S.Provisional Application No. 60/227,312, filed Aug. 24, 2000 and60/246,583, filed Nov. 8, 2000.

FIELD OF INVENTION

[0002] The present invention relates an improved system and methodologyfor substantially increasing the light output of a polarized opticalprojection system through the recovery of optical energy of an unusedpolarization.

BACKGROUND OF INVENTION

[0003] A liquid crystal display (hereafter “LCD”) is a known device usedto control the transmission of polarized light energy. The LCD may beeither clear or opaque, depending on the current applied to the LCD.Because of this functionality, projection systems commonly use an arraycontaining numerous LCDs to form an image source. In particular, theprojection system inputs high intensity, polarized light energy to theLCD array (also called an imager), which selectively transmits some ofthe inputted light energy to form a projection of a desired image.Because a single LCD is relatively small, numerous LCDs can be packedtogether into the array, thereby forming an imager that can produce ahigh resolution image.

[0004] As suggested above, a projection system must first polarize thelight input to the LCD. However, light energy from a light source, suchas a bulb, may have either p-polarization or s-polarization. Since thislight input to the LCD imager must be in one orientation (i.e., eitherp-polarization or s-polarization), the LCD projector generally uses onlyhalf of the light energy from the light source. However, it is desirablein projection systems to maximize the brightness and intensity of thelight output. In response, various methodologies have been developed tocapture the light energy of unusable polarization, to convert thepolarization of this captured light energy, and then to redirect theconverted light energy toward the LCD imager. These known polarizationrecovery methodologies involve creating an expanded beam of light inwhich the unused portion of the light (of undesired polarity) is sentthrough a half-wave plate to change the polarization and then recombinedwith the original polarized beam. Unfortunately, the implementation ofthese known methodologies requires complex, bulky systems, which usuallyinclude 2-dimensional lense arrays and an array of polarization beamsplitters. Furthermore, the known methodologies lose much of the lightenergy and, therefore, compromise the projector's goal of producing ahigh intensity output. As a result, there exists a current need for asimple, low cost, and compact polarization recovery system that operateswith high efficiency.

SUMMARY OF THE INVENTION

[0005] In response to these needs, the present invention uses awaveguide system to perform the polarization recovery function in an LCDprojection system. In particular, the present invention's waveguidepolarization recovery system both polarizes the input light energy foruse with an LCD imager and converts the polarity of unusable lightenergy to add to the illumination of the LCD imager. The compactpolarization recovery waveguide system generally includes the followingoptical components that are integrated into a single unit: (1) an inputwaveguide that inputs non-polarized light energy into the system; (2) anoutput waveguide that removes polarized light energy from the system;(3) a polarized beam splitter that receives the light energy from theinput waveguide and transmits light energy of a first polarization typeand reflects light energy of a second polarization type, and (4) a waveplate that modifies the polarization of either the transmitted orreflected light energy. The polarization recovery system also generallyincludes one or more mirrors that are positioned as needed to direct thetransmitted and/or reflected light energy to the output waveguide. Theinput and output waveguides may be shaped as needed by the projectionsystem. For example, either one or both of the input and outputwaveguides may be tapered as needed to produce a desired image.

[0006] In the waveguide polarization recovery system, the input andoutput waveguides are configured to have either a substantially parallelor a substantially perpendicular orientation. In configurations in whichthe input and output waveguides are substantially parallel, the outputwaveguide directly receives light energy transmitted by the beamsplitter. In this way, light energy enters and exits the polarizationrecovery system in substantially the same direction. Alternatively, theinput and the output waveguides may be positioned substantiallyperpendicular to each other such that the light energy exits thepolarization recovery system at a right angle from the direction itenters. In configurations having input and output waveguides ofperpendicular orientation, a mirror receives the light energytransmitted by the polarized beam splitter and redirects this energy by90° toward the output waveguide.

[0007] The polarization recovery waveguide system of the presentinvention combines the above-enumerated list of optical components intoa single, compact unit. In one embodiment, the waveguide polarizationrecovery system further includes one or more “gaps” of optically clearmaterial positioned between the optical components to encourage theoccurrence of total internal reflection that minimizes the loss of theoptical energy by the system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other advantages of the present invention will bedescribed in detail with reference to the following drawings in whichlike reference numbers refer to like elements:

[0009]FIGS. 1-4 and 6-10 are schematic diagrams that illustrate variousembodiments of the waveguide polarization recovery system of the presentinvention; and

[0010]FIG. 5 is a schematic diagram that illustrates a compactprojection device that uses one embodiment of the polarization recoverysystem of the present invention.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

[0011] As illustrated in FIGS. 1-4 and 6-10, the present invention is acompact waveguide polarization recovery system 10 having an inputwaveguide 20, a polarizing beam splitter (“PBS”) 30, a wave plate 40,which can be a half-wave plate, or a quarter-wave plate depending on theconfiguration, and an output waveguide 50. The waveguide polarizationrecovery system 10 generally further includes mirrors 60 as needed todirect the light stream between the input and output waveguides, 20 and50. The following discussion first summarizes several possibleconfigurations for the waveguide polarization recovery system 10 andthen describes the individual elements in greater detail.

[0012]FIGS. 1, 3, and 6 illustrate one configuration of the waveguidepolarization recovery system 10 in which the output light energy issubstantially parallel with the input light energy. In this embodiment,the input waveguide 20 introduces unpolarized input light at incidenceto the PBS 30. The illustrated PBS 30 transmits p-polarized light, andso the p-polarized portion of the input light energy continues throughin the same direction as the initial input while the s-polarized lightis reflected in a perpendicular direction to the initial direction ofinput. The half-wave plate 40 is positioned to receive the reflecteds-polarized light and convert it to p-polarized. Subsequently, mirror 60redirects the converted energy from the half-wave plate 40 back to theinitial direction of input. Both the transmitted light energy from thePBS 30 and the converted light energy from the half-wave plate 40 arerecombined in the output waveguide and mixed. As a result, the outputlight energy has a uniform intensity profile and is polarized. It shouldbe appreciated that an output of the opposite polarization may beproduced through the use of a PBS 30 that only transmits s-polarizedlight.

[0013]FIGS. 2, 4, and 7-8 illustrate another embodiment of the waveguidepolarization recovery system 10 that has an alternative configuration inwhich the output light energy is perpendicular to the original inputlight energy. As in the embodiment of FIG. 1, the input waveguide 20introduces unpolarized input light at incidence to the PBS 30.Furthermore, the PBS 30 performs the same function of transmitting thep-polarized light, and so the p-polarized portion of the input lightenergy continues through in the same direction as the initial inputwhile the s-polarized light is reflected in a perpendicular direction tothe initial direction of input. However, in the configuration of FIG. 2,one mirror 60 redirects the transmitted p-polarized portion of the inputlight energy by 90° toward the output waveguide 50. Furthermore, thereflected s-polarized light from the PBS 30 propagates once through aquarter-wave plate 40′, and a second mirror 60 then returns thereflected light energy to the quarter-wave plate 40′ for another pass.The second pass is also in the direction of the output waveguide 50.Because the reflected s-polarized light passes twice through thequarter-wave plate 40′, s-polarized light is shifted by a half-wave tobecome p-polarized twice with the mirror as shown. Again, bothp-polarized outputs will be mixed in the output waveguide, producing auniform intensity output. The embodiment of FIG. 2 requires only twooptical sections: A first section formed through the combination of theinput waveguide 20, the PBS 30, the quarter-wave plate 40′ and a mirror60; and a second section formed through the combination of the outputwaveguide 50 and a second mirror 60. Therefore, the system has a simpledesign and a relatively low cost. Positioning the output light energyperpendicular to the original input light energy also has the advantageof allowing a more compact projection system, as described in greaterdetail below.

[0014] In contrast to the above-described configuration in which thewave plate 40 modifies the light energy reflected by the PBS 30, otherconfigurations for the waveguide polarization recovery system 10position the wave plate to modify the light energy transmitted by thePBS 30. For example, FIGS. 9 and 10 illustrate configurations in whichthe half-wave plate 40 is positioned to receive light energy transmittedby the PBS 30. In the configuration of FIG. 9, the half-wave plate 40 isoptically positioned between a mirror 60 and the output waveguide 50.The half-wave plate 40 receives transmitted light energy that has firstbeen redirected by a mirror 60. Similarly, in FIG. 10, the half-waveplate 40 is placed between the PBS 30 and mirror 60. In this way, thetransmitted light energy from the PBS 30 is first repolarized beforebeing redirected toward the output waveguide 50. The configurations ofFIGS. 9-10 are advantageous because the input light energy only passesthrough the polarization layer of the PBS 30 once, thus reducing theloss of optical energy in the system 10. In contrast, theabove-described configuration of the FIGS. 2, 4, and 7-8 requires someof the input light energy to pass through the PBS 30 twice.

[0015] Elements of the Waveguide Polarization Recovery System

[0016] The various configurations of the waveguide polarization recoverysystem 10 use the same elements, which are now described in greaterdetail.

[0017] The input waveguide 20 is typically an integrator that collectsthe light from a light source, such as an arc lamp, and mixes the lightthrough multiple reflections to produce a more uniform intensity profileinto the waveguide polarization recovery system 10. Likewise, the outputwaveguide 50 is typically an integrator that collects the light from thewaveguide polarization recovery system 10 and mixes the light throughmultiple reflections to produce a more uniform intensity profile forillumination of the imager. The input waveguide 20 and the outputwaveguide 50 may be, for example, single core optic fibers, fusedbundles of optic fibers, fiber bundles, solid or hollow square orrectangular light pipes, or homogenizers, which can be tapered orun-tapered. In optical projection systems, the input waveguide 20 andthe output waveguide 50 are typically rectangular in cross-section tocorrespond with the shape of the imager and the final projected image.The input waveguide 20 and the output waveguide 50 wave can be made fromglass, quartz, or plastic depending on the power-handling requirement.

[0018] Either one or both of the input waveguide 20 and the outputwaveguide 50 can have an increasing or decreasing taper as needed forthe projection system. For example, FIG. 3-4 and 6-10 illustrateembodiments of the waveguide polarization recovery system 10 in whichthe input waveguide 20′ is a tapered rod with the input cross-sectionmatched to the area of the light source and the output cross-sectionrelated to the dimensions of a LCD imager. The final dimensions for theinput waveguide 20 may vary as needed to minimize stray light loss inthe optical projection system. Similarly, FIG. 8 illustrates anembodiment of the waveguide polarization recovery system 10 in which theoutput waveguide 50′ is also tapered. Tapering of the output waveguide50′ is advantageous because, depending on the performance parameters ofthe PBS 30, the wave plate 40, and the output requirements for theprojection system, polarization recovery may not always be done at thesame numerical aperture as the output aperture. The performances of thePBS 30 and the wave plate 40 are better at smaller numerical apertures,and as a result, advantageous increases in performance are achieved bytransforming the input light energy into a larger area with a smallnumerical aperture and then transforming the light energy back intolarger numerical apertures at the output of the output waveguide 50′.Overall, the tapering of the input wave guide 20 and the outputwaveguide 50 can be selected to match the overall performancerequirements of the projection system, and similarly, the input andoutput waveguides can be tapered in either direction.

[0019] The waveguide polarization recovery system 10 further includesPBS 30. The PBS 30 is a well-known optical element that transmits lightenergy of one polarization while reflecting light energy of a differentpolarization. Typically, the PBS 30 is a rectangular prism of opticallyclear material, such as plastic or glass, that has a polarizing coatingapplied to the diagonal surface. Alternatively, the PBS 30 may becomposed of a material that selectively transmit light energy dependingon the polarization of the light energy. However, it should beappreciated that there exist numerous alternative designs and types ofPBS, and any of these alternative PBS's may be employed in the waveguidepolarization recovery system 10 of the present invention. Because thePBS 30 is a well known and commercially available item, it is notdiscussed further.

[0020] Another element of the waveguide polarization recovery system 10is the wave plate 40. The wave plate 40 is an optically transparentcomponent that modifies the polarization of light energy that passesthrough the wave plate 40. The wave plate 40 typically changes thepropagating of light in one oaxis, thus changes the polarization. Thewave plate 40 may be either a half-wave or quarter-wave as needed by thespecific configuration of the waveguide polarization recovery system 10.Overall, the wave plate 40 is a well known and commonly available itemand will not be discussed further.

[0021] The waveguide polarization recovery system 10 may further includeone or more mirrors 60 as needed to direct the light energy through thewaveguide polarization recovery system 10. While mirrors are commonlyknown to be metal-coated glass surfaces or polished metal, the mirrors60 should not be limited to this common definition for the purpose ofthis invention. Instead, mirrors 60 should be considered any opticalcomponent capable of reflecting or redirecting light energy. Forexample, mirrors 60 may be prisms that use the angle of incidence tocapture and redirect light energy. For example, FIGS. 9 and 10illustrate a waveguide polarization recovery system 10 that has a prismto redirect light energy transmitted by the PBS 30 toward the outputwaveguide 50. For systems with small numerical apertures, total internalreflection at the prism can be used, and as a result, the coating is notnecessary.

[0022] In another preferred embodiment of the present invention,illustrated in FIGS. 6-10, the waveguide polarization recovery system 10further includes one or more optically clear areas, or “gaps,” 70between the other optical elements. The gaps 70 may be pockets of airleft between the optical components. The gap 70 can also be filled withlow index epoxy or other transparent material such that the totalinternal reflection still occurs, but the assembly of the componentswill be simplified. For example, FIG. 6 illustrates a configurationhaving gap 70 between the input waveguide 20 and the PBS 30. This gap 70ensures that light energy reflected by the diagonal PBS 30 is turned by90° toward the quarter-wave plate 40′ because total internal reflectionfrom the interface between the PBS 30 and the gap 70 prevents the lightenergy from returning instead to the input waveguide 20 and exiting as aloss. The waveguide polarization recovery system 10 in FIG. 6 also hasother gaps 70 to promote total internal reflection between the differentoptical elements. Similarly, FIG. 7 illustrates a waveguide polarizationrecovery system 10 in which gaps 70 have been added to a polarizationrecovery system with a tapered input waveguide 20 and perpendicularlyconfigured output waveguide 50 illustrated in FIG. 4. Again these gaps70 increase the efficiency by encouraging total internal reflectionbetween the optical components. As illustrated in FIGS. 6-7, the gaps70, while increasing the efficiency of the system, cause the waveguidepolarization recovery system 10 to become more complex with an increasednumber of discrete parts.

[0023] In the above-described configurations of FIG. 9-10, the gaps 70further serve the purpose of improving the performance of the prism 60′that serves as a mirror to direct the light energy toward the outputwaveguide 50. In particular, the gap 70 is needed between the PBS 30 andthe prism 60′ such that the light reflected from the hypotenuse of theprism 60′, back toward the PBS 30, hits this interface of the gap 70 andis internally reflected toward the output waveguide 50. In this way,efficiency of the system is improved by minimizing loss.

[0024] The performance advantages of the gaps 70 may be furtherincreased through the use of anti-reflection coating on both surfacessuch that the transmitted light suffers minimal loss.

[0025]FIG. 5 illustrates a projector 100 that employs the waveguidepolarization recovery system 10. The projector 100 consists of a lightcollecting system 110, which in this illustrated example has twoparaboloid reflectors and a retro-reflector that increase the output byreflecting the light from a light source 120 back into itself. The arcof the light source 120 is placed at a focus of the first paraboloidreflector and the proximal end of the input waveguide 20 is at the focusof the second paraboloid reflector. It should be appreciated that thislight collection system 110 is provided merely for illustration, andmany other light collection systems are known and may be used. Likewise,the light source 120 may be an arc lamp, such as xenon, metal-halidelamp, HID, or mercury lamps, or a filament lamp, such as a halogen lamp,provided that the system is modified to accommodate the non-opaquefilaments of the lamp.

[0026] Within the illustrated projector 100, the input waveguide 20 is atapered light pipe that is designed to match the light input collectedfrom the light collecting system 110 to the optical needs of an LCDimager 150. As described above in FIG. 4, the light output of the inputwaveguide 20 is polarized by the PBS 30 and the other polarization isrecovered by the quarter-wave plate 40′. The output waveguide 50 thendirects the polarized optical energy toward the LCD imager 150. In thiscase, the light output in the output waveguide 50 is then incident intoa second PBS 130 whose orientation is matched to the polarization of theincident light to minimize the loss. A color wheel 140, or other type ofcolor selection system, and the reflective LCD imager 150 create theprojected image by the projection lenses 160 in a traditional manner. Asshown in FIG. 5, the number of optical elements is minimal and, as theresult, the cost for the projector is relatively low.

[0027] It should be appreciated that the waveguide polarization recoverysystem 10 may be used in other types of projection systems. For example,the projector may also use two or three imagers 150 to define theprojected image. The imager 150 may also be a reflective display usingliquid crystal on silicon (“LCOS”) technology, or any other type ofsystems that requires polarized light. Similarly, the color wheel 140can also be replaced by electrically switchable color system withoutmoving parts or other known color projection systems.

[0028] While the invention has been described with reference to thestructures disclosed herein, it is not confined to the details setforth, and this application is intended to cover such modifications orchanges as may come within the purposes of the improvements or the scopeof the following claims.

1-54. (Cancelled)
 55. A light guide comprising at least first and secondlight guide sections and at least one light coupling element between thelight guide sections, the light guide sections each having an entranceaperture, an exit aperture and a guide wall defining a light guidingdirection, the light coupling element being composed of an opticallytransparent material having an entrance face with a TIR surface, and anexit face with a TIR surface, the entrance face adjacent to the exitaperture of the first light guide section and extending in the lightguiding direction of the second light guide section, and exit faceadjacent to the entrance aperture of light guide section and extendingin the light guiding direction of the first light guide section.
 56. Thelight guide of claim 55 in which the light coupling element iswedge-shaped and has an internally reflecting face extending between theentrance face and the exit face.
 57. The light guide of claim 56 inwhich the cross sections of the light guide sections have the crosssectional shape of an even-numbered polygon.
 58. The light guide ofclaim 55 in which at least the first light guide section is a solidelement of an optically transparent material, and the entrance face oflight coupling element adjacent to the exit aperture of the solid lightguide section is spaced a distance apart from the exit aperture of lightguide section, to form a space between the entrance face of lightcoupling element and the exit aperture of the solid light guide section.59. The light guide of claim 58 in which the second light guide sectionis a solid element of an optically transparent material, and the exitface of light coupling element adjacent to the entrance aperture of thesolid light guide section is spaced a distance apart from the entranceaperture of light guide section, to form a space between the exit faceof light coupling element and the entrance aperture of the solid lightguide section.
 60. The light guide of claim 58 in which the space isfilled with air.
 61. The light guide of claim 58 in which the space isfilled with an optically transparent material having an index ofrefraction less than that of the material of the light coupling element.62. The light guide of claim 58 in which the distance d between theentrance face of light coupling element and the exit aperture of thesolid light guide section is substantially the same across the width ofthe space.
 63. The light guide of claim 55 in which at least the firstlight guide section is a hollow conduit having a guide wall with aninterior reflective surface.
 64. The light guide of claim 63 in whichthe second light guide section is a hollow conduit having a guide wallwith an interior reflective surface.
 65. The light guide of claim 55 inwhich the entrance and exit faces of light coupling element form a rightangle.
 66. The light guide of claim 64 in which the guide walls form anobtuse angle.
 67. The light guide of claim 56 in which the reflectivesurface is a polarizing surface, reflecting a first polarizationdirection and transmitting a second one.